I have used the Koni-Omega system for years. The lenses, even though produced back in the '70's or earlier, produce superb results; enlargements to 16 x 20 are no problem at all. I use a 58mm, a 90mm, and a 180mm, and they all produce good results. There is also a rather rare 135mm which is occasionally listed on E-Bay; usually the final price is quite high. The strength and weakness of the system involves the backs. On the plus side is the quick interchangeability of magazines--really handy during rapidly-changing events. The negative is that the backs seem to be the mechanical weak point of the system. Faulty film spacing is not unusual with older, well-used backs. Rapid film advance is encouraged by the design, but it's easy to be a little too vigorous and eventually cause some internal damage.
The lock-on sync cords are great--I've never had a failure. The ingenious system of double-exposure prevention, blank- frame prevention interlocks used in changing lenses /magazines, and the sheer sturdiness of the camera are strong assets.
Be aware that there are two different film-back designs. The "flatter" design is less desirable. The "beveled" design is compatible with the interchangeable magazines and will also fit the Koni TLR. It's probably best to get a Rapid M, a 100, or a 200 rather than older models.
The worst thing about the Koni-Omega is that it's no longer being produced; the best things are that it is very durable and there are no batteries in sight! Now, if only someone would tool up to make compatible backs/magazines with the weak points of the design eliminated. . .

I've got the 100 collecting dust somewhere, with 58mm and 90mm lenses. I find it a little too bulky for my liking, although the hand grip for it works pretty well. It's just a pain (or rather arm fatigue) to hand hold it for a significant amount of time.

Otherwise, it's pretty solid, and rapid between shots, although I've never really needed the speed.

I've only had one problem with it. With the 58mm lens on, if the camera is pointing up sharply (or maybe down, I can't remember), something about the focusing mechanism gives problems to the back (again, I have the 100 version, not sure about the others). When focusing from up close back to infinity, part of the camera would push backwards towards the back, preventing it from going all the way back to infinity. At first, I tried to force this and it wound up bending something in the film holder so that it was no longer flush with the back of the camera. This only ruined 1 roll of film, and I was able to bend back the part pretty easily. Anyways, you can get around this problem by pointing my camera down (or up, if the problem occurs in the other direction, I can't remember), and then focusing back to infinity.

If I remember correctly, I found the users manual for the 100 and 200 online somewhere.

The first medium format camera I used for professional work was a Koni-Omega Rapid. I loved the camera. Interchangeable lenses, interchangeable backs, fast to use. The first aerial work I ever did was with that camera. The only problem we ever had was with the backs and the film advance mechanism. Easy to get fixed at that time because they were still in production. Today?? If they need parts?? Lenses were sharp with good contrast. If the camera is in working condition, lenses are okay, and the whole thing is not too expensive, it might be worth a go as they certainly were capable of taking good photos.